Gas concentration measurement by light absorption makes use of the nature of molecular and atomic gases to absorb portions of the electromagnetic spectrum that correspond to energy transitions of the absorbing gas species. There are several methods for using light absorption to measure gas concentration of species in a gas mixture.
Cavity ringdown spectroscopy (CRDS) is a gas sensing technique in which an optical cavity is formed by two or more highly reflective mirrors. Typically, CRDS-based measurements can provide 10-1000 times improvement in sensitivity compared to more conventional absorption spectroscopy measurements. In CRDS, laser light is directed into the cavity at one mirror and aligned to cause the light to propagate in the cavity between the mirrors in a repetitive path. Optical output at one or more mirrors is monitored with a light sensor. The laser is either pulsed or made to cease entry into the cavity after a period of time, following which the signal recorded at the light sensor is a decaying or ringdown signal, which decreases in time in a known way that is related to the concentration of gas species in the cavity.
Integrated cavity output spectroscopy (ICOS) is another gas sensing technique that can be used to measure gas concentration of species in a gas mixture. This particular technique provides an enhanced cavity sensitivity as compared to convention CRDS and uses a less complex setup than conventional CRDS. In CRDS it is generally desirable to only excite the TEM00 mode of the laser into the cavity. This is because the transverse modes sample a greater portion of the mirror surface. Inhomogeneties in mirror reflectivity lead to variation in ringdown time constants. With ICOS, the transmitted output of the cavity is time-integrated to provide an absorption spectrum as the wavelength is scanned through the region of interest. The spectrum is then converted to absorbance through the Beer-Bougert-Lambert Law.
Other known measurement techniques are also described in U.S. Pat. No. 5,912,740 to Zare et al. and U.S. Pat. No. 6,795,190 to Paul et al., both of which are hereby incorporated herein by reference in their entirety. In the '740 patent ring-shaped resonant cavities for spectroscopy are provided which allow a reduction in optical feedback to the light source, and provide information on the interaction of both s- and p-polarized light with samples. A laser light source is locked to a single cavity mode. An intracavity acousto-optic modulator may be used to couple light into the cavity. In the '190 patent, a method is described which introduces a continuous-wave light beam into the cavity using an off-axis cavity alignment geometry to systematically eliminate the resonances commonly associated with optical cavities, while preserving the absorption signal amplifying properties of such cavities. This reduces the complexity of the apparatus considerably compared with other optical cavity-based absorption methods when applied in conjunction with either CRDS or ICOS.